The principle of scanning objects with high energy radiation such as x-rays or gamma-rays is widely employed for example in the security industry. The invention is discussed below by way of example in such a context but might also be employed in other areas, for example, without limitation, medical imaging, imaging for quality control purposes or the purposes of determining the integrity of the structure, or the like.
Security scanning is particularly directed at the identification of undesirable materials or objects, and especially explosives or weapons, in airline baggage. Airport security must in particular ensure that no explosive material is allowed on board any aircraft. Various strategies are employed in order to achieve this goal, however one of the most important is the screening of hold baggage using x-ray machines that have automated explosive detection capability.
Many techniques and technologies have been proposed for creating machines that can automatically detect contraband materials. Example include x-ray diffraction, x-ray scattering, Mossbauer spectroscopy and NQR. However the only technology currently accepted by the TSA (Transportation Security Agency: US government) is one that utilises a computerised tomography (CT) approach to identifying materials.
Whilst these known apparatus and methods perform the desired role, there are a number of areas that need to be improved especially in relation to false detection rates, for example between 15 and 30% of submitted articles. Any articles that are deemed to be suspicious have to go on for further screening and so a high false alarm rates greatly limits the productivity of these machines.
Prior art machines take scans of the baggage at different angles and by looking at the x-ray transmission image they analyse intensity of the beam produced by the materials within the baggage. To maintain adequate throughput the scanning processing is typically much reduced when compared to medical applications where a volume rendered image is desired. The method relies on the building up of an empirical database of suspect material and a comparison of the intensity of information produced by the materials in the baggage obtained by numerical analysis of the slices obtained by the reduced CT scan with the database thus making it possible to identify potentially dangerous material. However, there are limitations to the extent to which such data can characterise material as such.
The transmission of x-rays through a material can be given by the exponential attenuation law, as follows:I/Io=exp[−(μ/ρ)ρt]  (1)                Where μ/ρ=Mass attenuation coefficient, a material constant which is characteristic of the weighted elemental composition of a material        I=Final intensity        Io=Initial intensity        ρ=density of the material        t=thickness of the material        
The approach taken in the CT methodology is to vary the path taken by the x-rays through the baggage. This effectively changes the thickness of the material, the term ‘t’ in the equation. Thus by looking at the variation in the x-ray transmission deductions can be made about the mass attenuation coefficient and the density of the material. These two parameters are characteristic of different materials and so materials identification becomes possible.
One of the problems is that more than one material will frequently be in the system path and so the results of combinations of materials needs to be determined. This requires an extensive database of information and sophisticated algorithms.
In addition, as the beam direction changes, the relative amount of each material in the x-ray path will also change. This adds yet more uncertainty.
Finally the density term and mass coefficient term are convoluted adding further difficulties.
It can be seen that there are many reasons for the high false alarm rates given by CT and like methods.
U.S. Pat. No. 5,367,552 is an early example of a system using CT type scanning to detect explosives. This reference illustrates the reduction in typical scans used by the technology in an explosives detection field.
A number of sources have suggested use of the above relationship numerically to derive mass attenuation terms as an aid to materials identification.
U.S. Pat. No. 5,319,547 describes using two monochromatic x-ray sources and looking at relative ratios in order to identify materials from the differences in their transmission values.
U.S. Pat. No. 5,768,334 uses a single x-ray source but altered the output energy by spinning a filter wheel of different materials through the beam. It then described use of comparative techniques to determine whether the sample under inspection contained any of the component materials in the filter wheel. The technique is limited by the number of materials that can be placed in the filter wheel and it is also slow as a signal needs to be obtained across each part of the filter wheel.
U.S. Pat. No. 6,018,562 uses multiple x-ray tubes running at different powers with suitable multiple detectors. The broadness in the energy of the beams in each tube means that the precision in the determination of the mass attenuation coefficient is limited which also compromises the ability to distinguish similar materials.
WO2005/009206 tackled the problem of gaining x-ray photons of different energies by varying the power going into an x-ray source. This has the advantage of being able to produce x-rays at many different power levels and hence at different energy spectrums. However the problem of the width of the energy band remains. It is also a slow approach as spectrum across the power source range defined needed to be collected at each point of the sample.
All of these methods have in common the selection of energy upstream of the detector, for example at the source, by provision of multiple sources and/or additional filters, multiple detectors tuned for different energies etc.
X-Ray absorption has also been used for some time as the basis for screening objects to create some form of representational image of the contents or components thereof relative to each other in three-dimensional space. The thicker or more dense an object is then the more it will attenuate an x-ray beam. By use of suitable detectors and a suitable source, radiographs of an item under screening in the form of images based on the absorption of an object or set of objects can be generated. In airline security applications, the principle is encountered in particular in relation to hand baggage scanners. X-ray imaging might also be used in principle as a supplementary system for hold baggage (the reduced CT scan of the detection application being limited as regards imaging capability) but this is less common.
This apparatus may be limited in that it tends to give limited information about the material content. At its simplest, all that is being measured is transmissivity. The detector merely collects amplitude information. In most practical systems even this is measured indirectly. A typical linear array x-ray detector comprises in combination a scintillator material responsive to transmitted x-rays, which is then caused to emit lower frequency radiation, and for example light in or around the visible region, in combination with a semiconductor detector such as a silicon or gallium arsenide based detector which is responsive to this lower frequency radiation.
However, it is known that the absorption properties of any material can vary with energy, and that the amount by which the absorption properties vary depends in particular on atomic number (in part at least because different absorption effects predominate). This has led to development of dual-band or dual-energy detectors which are capable of separately differentiating, at least to some degree, between low- and high-energy bands. Such a dual-energy sensor typically comprises a sandwich pair of semiconductor photodiode rays or the like, in conjunction with a low-energy and a high-energy scintillator, such that the respective detectors detect transmission of low-energy and high-energy x-rays. The differential absorption effect is exploited by the dual energy detector to differentiate generally between objects having lower and higher atomic number elements predominating.
When exploited as part of a security or material imaging system, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category. The practical implications of this have led to its use in the security industry, and for example in airport x-ray scanners, either to create separate images of metallic items inside luggage (to reveal items hidden metal weapons, such as guns, and knives) or to identify plastic explosives.
Such a system is of limited effectiveness. For example, considering use of the principle in explosives scanners, it is true that most explosives are dense organic materials usually high in nitrogen content. There is therefore some limited merit in the use of dual energy detectors but it is far from being a precise explosive detector since many other items in luggage, such as soaps, creams, leather goods etc are also such dense organic materials.
A dual energy system thus confers only limited information about composition. The organic/inorganic division is crude and approximate. Conventional detectors do not give any real information about material composition. At best, crude presumptions are made based on the presence or otherwise of x-rays within two distinct bands of the spectrum, usually in conjunction with a radiograph which is based on the shape of items and their proximity to other objects.
U.S. Pat. No. 4,247,774 represents a general reference to the use of a dual-energy detector system in relation to computer assisted topography in a medical, imaging application.